Solvent extraction is used inter alia for removing selected metals from aqueous solution. The process typically comprises two stages, extraction of the metal values from aqueous solution by an organic extractant and stripping of the metal values from the metal loaded organic extractant by an aqueous stripping solution. In the extraction and stripping stages, each stage consists of a mixer (or series of mixers) and a settler, where metal exchange from one phase to the other takes place. The number of extraction and strip stages required to recover a desired metal value can be determined by metallurgical testing of the solutions in question. The mixer-settler of each stage consists of a mixer (or mixers) section and a settler section. In the mixer section, the aqueous solution containing the metal value is mixed with an organic extractant which selectively combines with the metal to be extracted. During an extraction stage the aqueous solution and organic are mixed to form an emulsion to provide as much as possible contact between the organic and aqueous phases to maximise extraction of the metal into the organic phase. In a stripping mixing stage, the metal value is re-extracted from the organic phase by the aqueous stripping solution. When an emulsion from either the extraction or strip stage enters its settler section, the emulsion from the mixer is introduced into the settler where the organic and aqueous phases are allowed to separate with the organic forming a layer above the aqueous phase. The two phases are in constant flow in the settler. Separate overflows for the organic and aqueous phases are provided at one end of the settler so that the organic is retrieved for subsequent treatment in the next stage of extraction or stripping. The exiting aqueous solution also reports to its subsequent destination in the process. Typically, both the organic and the aqueous phases flow in the same direction in the settler, i.e. there is co-current flow in the settler.
A problem with the above conventional arrangement is that impurities present in the aqueous solution, referred to as "crud" form a layer between the organic and aqueous layers in the settler and a so- called "crud-run" is experienced when the crud accumulates at the organic overflow and exits with the organic, which causes problems.
It is accordingly an object of the present invention to alleviate the abovementioned difficulty.